Hydroisomerization process



CL Z7, 1959 c. w. MoNTGOMl-:RY ETAL 2,910,523

HYDROISOMERIZATION PROCESS Filed May 8, 1956 3 Sheets-Sheet 1 i zj-'7.1.

,f7-roe Navi- Oct 27, 1959 c. w. MoNTGoMERY Erm. 2,910,523

HyDRoIsomzRIzATIoN PRocEss Filed May 8, 1956 3 Sheets-Sheet 2 zo V v L K h lbf". *q o v ak /C 0.5 /v Q IN V EN TUR- ann: MVa/:mrfry 0t- 27, 1959 c. w. MoN'rGoMERY Erm.

HYDROISOMERIZATION PRCESS :s sheets-sheet s Filed May 8, 1956 J7 N .y

IN V EN TOR5 United States Patent 1 2,910,523 HYDROISOMERIZATION PROCESS Charles W. Montgomery, Oakmont, William C. Starnes,

Pittsburgh, and Robert C. Zabor, Glenshaw, Pa., as-

signors to Gulf Research & Development Company,

Pittsburgh, Pa., a corporation of Delaware Application May 8, 1956, Serial No. 583,497 5 Claims. (Cl. 260-683.65)

This invention relates to a process for isomerizing npentane to isopentane in the presence of hydrogen and a solid catalyst. More particularly, it relates to an improvement in the control of the operating conditions of such a process.

The conversion of n-pentane to isopentane has great commercial importance in the petroleum and chemical industries. It is important in petroleum reiining because isopentane is valuable as a high octane number component of gasoline. Isopentane is also valuable as a chemical reactant, for example, as a raw material in the production of isoprene. The present process therefore is valuable for increasing the isopentane content of a renery pentane fraction to be used as a gasoline component and for producing substantially pure isopentane as a chemical reagent or raw material.

A new and improved process for isomeriz'ing rx-pentane to isopentane has been described in the patent application of William C. Starnes and Robert C. Zabor, Serial No. 508,980, filed May 17, 1955, now U.S. Patent No. 2,831,908. The process is characterized by such features as the use of a supported platinum catalyst, the use of a low hydrogen concentration in the feed and a high space velocity as compared With naphtha reforming operations. The novel process results in an unusually high spacetime-yield of isomer product and a high process etliciency. The present invention provides a further improvement in the Starnes and Zabor process. The process of the invention is particularly concerned with the control of olefin concentration of the product so as to avoid rapid aging of the supported platinum catalyst.

The process of the invention in general comprises contacting a feed mixture of n-pentane and hydrogen with a supported platinum catalyst at a temperature from 700 to 1000 F., and a liquid-hourly space velocity of at least tive volumes of n-pentane per volume of catalyst per hour, the concentration of hydrocarbon in the feed ditions.

1n describing our invention more fully, we will refer to the drawings, of which Figure 1 is a diagrammatic illustration of apparatus with which the process of the invention can be carried out; and

Figures 2 and 3 are graphic representations of the relationships between olefin concentration of the isomerization product, catalyst life and space-tirne-yield of isopentane in the isomerization of n-pentane.

We have discovered that when n-pentane is isomerized to isopcntane the reaction product contains oleiinic hydrocarbons, principally pentenes, the concentration of which in the isomerization product depends upon the reaction Variables of temperature and pressure and the concentration of hydrogen in the charge stock to the process. We have further discovered that the concentration of olenic hydrocarbons in the isomerization product is related to catalyst aging rate. We have carried out a number of runs in the hydroisomerization of n-pentane which demonstrate these facts and the principles of our invention. One series of such runs was carried out as described in the following example.

EXAMPLE I The catalyst employed was a platinurn-alumina catalyst which consisted essentially of 0.57 weight percent platinum, 0.64 weight percent chlorine, and the balance alumina. The surface area of the calcined catalyst was 295.7 square meters per gram. The fixed bed isomerization reactor contained 60 cc. or 48.9 grams of the platinum-alumina catalyst. Before placing the reactor containing the freshly calcned catalyst on-stream, the catalyst was heated to 1000 F. in a stream of hydrogen for a period of several hours and the reactor was pressured to the reaction pressure of 500 pounds per square inch gauge. For each run the reaction temperature was Table l Run No 1 2 3 4 5 6 7 8 9 10 11 12 Throughput age at start of cycle 247 259 268 288 305 320 343 370 398 447 478 512 Temperature, F 830 830 830 828 831 828 83l 831 829 829 829 829 Pressure, p.s.i.g, V500 500 500 500 500 500 500 500 59D 500 500 500 Space Velocity, v0l./l1r./vol 6. 6 1. 3 4. 8 8. 4 3. 0 `6. 6 9. 0 9. 0 9. 6 10.1 10. 6 11.3 )lydrogen feed rate, scf/h 2. 44 Y 4. 64 3.09 1. 59 3. 90 2. 35 1. 33 1. 33 1.08 0. 79 0.51 0. 28

arge:

Mole fraction hydrocarbon (Nac) 0. 54 0.11 0.41 0. 70 0.25 0A 55 0.75 0.75 0. 80 0.85 0. 90 0.95

Recovery, percent by weight of charge:

Liquid product 97.4 97. 7 98. 4 98. 5 94. 1 98.0 98. 2 99. 2 99. 7 99. 5 98, 2 99. 1 Gas:

C, to C4 incl 0.5 4.6 0.7 0 4 1 0 0.4 0 3 0.3 0.4 0.3 0.3 0.4 5+ 0.1 3. 2 0. 3 0 1 l 7 0.2 0 l 0.03 0. 03 0.01 0.02 0.03 Total l 98,. 0 105. 5 99. 4 99. 0 96. 8 98. 6 98. 6 99. 5 100. l 99. 8 98. 5 99. 5

Product composition:

Mol. percent of charge:

V 4 in 1.7 V9.2 2.6 2.2 3.4 1.7 1.5 1.9 2. 5 1.9 2.0 3 5 ISO Pentane. 38. 5 36. 2 39. 2 39. 3 33. l 34. 0 38. 0 37. 3 35. 0 34. 6 38. 9 36. 3 n-Pentane 59. 6 57. 5 58. 1 57. 5 64. 3 64. 0 60. 3 60A 3 61. 3 63. 0 58. 3 59. 4 Cyclopentane 1. 2 1. 4 1. 3 1. 9 0.8 1. 1 1.0 1. 4 l. 2 1. 4 1.8 l. 4 2,2-din1ethylbutane. 0. 9 0. 7

Total 101. 0 104. 3 101. 2 100. 9 101. 6 100. 8 100. 8 100.9 100. 9 100. 9 101.0 101.3 M01. percent isomcrizati 34; 1 3l. 6 34A 9 35.0 28. 2 29. 2 33. 6 32. 8 30. 3 29. 8 34. 5 3l. 7 Mol. percent conversion 34. 4 36. 7 36. l 36. 7 2E). 3 29. 6 33. 7 33. 7 32. 6 30. 7 35. 9 34.7 Percent eliciency 99. 1 86.1 96. 7 95. 4 96. 2 98. 6 99. 7 97.3 92. 9 97. 1 96. l 91. 4 spacetime yield 1C; vol./hr./vol 2.08 0.38 1. 58 2. '74 0.78 l- 79 2. 80 2. 75 2. 70 2.84 3. 40 3. 34 Oletin content of liquid product, Mole percent- 0. 27 0. 54 0. 36 0. 59 0.68 0. 8G 1. 35 2.02

Patented Oct. 27, 1959.

`about 830 F. and the pressure was 500 pounds per square inch gauge. The hydrocarbon content of the feed mixture Was predominantly n-pentane and consisted essentially of 90.8 percent n-pentane, 7.8 percent isopenture composition which produces a concentration of 1.5 mol percent of olens in the product is about 0.92 mol fraction o-f hydrocarbon.

We have carried out another series of hydroisomerizarapid catalyst aging begins. As the curve shows, when the olen concentration of the product is approximately 1.5 mol percent, the space-time-yield of isopcntane is substantially at a maximum but the peak of curve A at which rapid catalyst aging occurs is not yet reached. Under tane, and 1.4. percent cyclopentane. A different feed 5 tion runs with a supported platinum catalyst diiferent hydrogen concentration was used for each run. Table I from that used in Example I and the results of these "above records the process conditions and product data runs conrrn the results discussed above in connection 'for each run of this series. with Example I. Details of this second series of runs Certain of the product data from Table I are plotted are described in Example II below. lin Figure 2 of the drawing. Curve A in Figure 2 is a 10 EXAMPLE H plot of space-time-yield of isopentane against the mol fraction of hydrocarbon in the charge mixture for the The ca talys Used Was e Pelleled PaUnUIn-alurnlna Carseries of runs described in Example I. Curve A shows alyst Whleh oonslsted essenfleuy of 0-37 Pereent Planan increase in space-time-yield of isopentane as the mol num 024 Percent ohlorlne, 058 PerCent fluorlne and the yfraction of hydrocarbon in the charge mixture increases balance alumma- The Surface area of the fresh eaelyst up to a value of about .94 mol fraction of hydrocarbon Was 17d-1 square rne'fers Per gram The nXed bed lso- (corresponding to a hydrogen Concentration of about merization reactor contained 60 cc. or 32 grains of the 75 standard cubic feet per barrel of hydrocarbon). oatalyst- Tire Catalyst had been Used PreVlonslY for Further increase of the hydrocarbon concentration rehydrolsornerlzarlon of n-Pentene for e total nronghlut sulted in rapid deactivation or aging of the catalyst. This 20 of 456 Velght mts 0f npentane Per umt 0f catalyst and was demonstrated by roo 12 carried oot at a mol fracafter this use the catalyst had been regenerated by burntion of hydrocarbon of 0 95 This mn is plotted as ing with a nitrogen stream containing small amounts of point B in Figure 2 and lies considerably below the einer e maximum ternPeradre of 900 F following curve traced by the space-tinie-yields for the other runs. Whloh the Catalyst Was heated 1n hydrogen to 1000 F at The deactivation of the catalyst at high hydrocarbon a Pressure of 500 Pounds Persqnare lneh gouge, e hydro` concentration of the feed is further shown by the circle gen rare of 1-0 standard eublo feet of hydrogen Per bar B plotted in Figure 1. This represents a space-time-yield rel Per hour over a Perlod of 4 hours The Predonnof about 2.7 volumes of isopentane per volume of catnen'dy n-Pen'fane Charge 1n dns SerleS Of Inns WaS the alyst per hour which was obtained in a run with a catalyst saine as nsed 1n the runs of Exemple I For each run and under conditions substantially the same as those of the reaction temperature Was abolir 810 F and d le Example I but charging a feed mixture with a hydra- Pressure Was 500 Pounds Per square lnoh geldge- A dlf' -carbon mol fraction of 0.97. The space-time-yields repferent hydrogen: hydrocarbon reno for the Charge Was resented by points B and B' notonly were lower than used for each run. Table II records the process condispace-time-yields obtained at the points along curve A tions and the product data. for each of the mns.

' Table 11 Run No 13 14 15 16 17 18 19 20 21 22 23 94 Throughput age at start of cycle.. l 622 639 661 687 715 744 775 833 862 894 927 956 Temperature, F- 810 810 810 810 810 811 809 810 810 810 811 810 Pressure, 9.9.1.9--- 509 500 500 500 500 500 500 500 500 500 500 500 space velocity, v0 /v 4.8 6.6 8.4 8.9 9.6 10.2 10.2 8.4 10.8 10.8 10.8 8.4 Hydrogen feed rate, s.c.f./hr 3.13 2.45 1.55 1.33 0. 94 0.74 0.74 1.60 0.50 0. 50 0.49 1.60 Charge:Molefractionliydrocarbon (Nnc) 0.40 0.54 0.71 0.75 0.82 0.86 0.86 0.70 0.90 0.91 0.91 0. 70

Recovery, percent by weight of charge:

Iiquid product 98.0 99.4 98.8 97.9 98.5 99.3 98.2 99.2 99.1 98.9 98.8

BSI

Clto Cinisi 0.4 0.3 0.2 0.2 0.2 0.2 0.2 0.3 0.2 0.2 0.2 0.3 05+ 0.3 0.2 0.1 0.94 A 0.1 0.03 0.03 0.1 0.01 0.04 0.02 0.1

Total 96.4 98.5 99.7 99.0 98.2 98.7 99.5 98.6 99.4 99. 3 99.1 99.2

Product composition:

Mol. percent of charge:

Ci-Ciinei 1.5 1.7 1.1 1.0 1.1 1.7 0.7 0.9 1.5 1.4 1.2 1.0 iso-Pantano 38.9 38.5 38.9 40.6 41.8 44.7 42.7 38.7 44.6 43.5 42.2 34.5 n-Pentane 59.2 59.4 59.0 57.4 66.1 53.1 55.1 59.3 53.1 53.8 55.3 63.4 Cyeiopeutane 1.1 1.2 1.5 1.7 1.5 1.3 1.7 1.5 1.4 1.7 1.4 1.5 2,2-dimcthylbutane 0. 1 0. 2 0. 4

Total 100.7 100.8 100.5 100.7 100.5 100.8 100.2 100.4 100.6 100.6 100.5 100.4 Mol. percent isomerzation. 34.5 34.1 34.5 86.4 37.7 40.9 38.7 34.3 40.7 39.6 38.2 29.7 Mol. percent conversion--. 34. 9 34.7 35. 1 36. 9 38. 3 41. 6 39. 4 34. 8 41. 6 40. 8 39. 2 30. 3 Percent efficiency 98.9 98.3 98.3 98.6 98.4 98.3 98.2 98.6 97.8 97.1 97.4 98.0 spacetime yield 101 vo1./hr./vo1 1. 53 2.10 2.81 3.03 3.39 3. 86 3.65 2. 68 4.06 3.99 3.90 2. 33 Olencontent ofllquid product, mole percent.. 0.32 0. 0. 45 0. 50 0.63 0.68 0.36 0.72 0.77 0.81 0.36

but the space-time-yield was rapidly declining during the v Figure 3 of the drawing plots certain of the product throughput periods for which the products represented data from Table II. Curve E in Figure 3 is a plot similar by points B and B were collected. to curve A in Figure 2 of the spacc-timc-yield of iso- Curve C in Figure 2 is a plot of olen concentration of 65 pentane against the composition of the charge mixture in the product against m01 fraction of hydrocarbon in the terms of mol fraction of hydrocarbon. Curve F, like charge mtxture for each of the runs. A comparison of curve C of Figure 2 is a plot of olen concentration of lcurves A and C of Figure 2 shows that a marked increase the product against the composition of the charge mixture. 1n the rate of oleiin formation occurs at substantially the Curve F shows that an olefin concentration in the prodn same charge mixture hydrogen concentration at which uct of about 1.0 mol percent or less corresponds to the mol fraction of hydrocarbon in the feed at which rapid deactivation of the catalyst occurs under the con ditions of these runs and with the particular catalyst used. The two circles G represent the spacetime-yields obtained in runs 22 and 23 in which the charge mixture the operating conditions of these runs the charge 75, had a hydrocarbon mol fraction of 0.91. These two demonstrate the catalyst deactivation that occurs When the hydrogen concentration of the feed is too low. G represents the spacetime-'yield obtained in run 24 using a feed composition similar to that of runs 15 and 20 but performed after the catalyst had been deactivated by operating with an excessively high hydrocarbon concentration for the feed. At point G the spacetimeyield was only about 2.3, considerably below the space-timeyields obtained at the points along curve E. This shows that the deactivation of a catalyst caused by using an excessively high hydrocarbon concentration for the feed cannot be restored merely by decreasing the hydrocarbon concentration below the concentration which initially caused rapid deactivation.

In addition to our discovery of the correlation between olefin content of the isomerization product and catalyst aging rate, we have discovered that theoretical olelin concentrations obtained by thermodynamic calculations 'on the pentanes-pentenes-hydrogen system agree almost identically with the data obtained in actual nuns and we 'have'developed a mathematical relationship for variables of the pentane isomerization process such `as temperature, pressure, hydrogen-pentane feed ratio and olefin con centration of the product. We have found that the required hydrogen-pentane mol feed ratio required to pro duce any specified mol fraction of pentenes in the product is expressed by the following equation:

wherein r is the mol ratio of hydrogen to pentane in the feed; .t is the mol fraction of pentenes in the product; a is the equilibrium constant for the isomerization of n-pentane to isopentane; and b is the quotient of the sum of the equilibrium constants for the dehydrogenation of vn-pentane to each of the pentenes divided by the total absolute pressure in atmospheres.

We have calculated olefin yields for the runs of Examples l and` ll using equation (l) and have plotted them in Figures 2 and 3 of the drawing. They are represented by the small circles such as D in Figure 2 and H in Figure 3 which lie substantially along curves `C and F. The plots show that the calculated ole'lin yields agree substan tially identically with the yields obtained in the actual runs of Examples l and ll. Thus, in accordance with the invention, we can use Equation 1 to determine the mini mum hydrogen concentration that can be used with a particular` catalyst under any given hydroisomerization conditions without deactivating the catalyst rapidly.

Example il shows that if the hydrogen is reduced below a minimum hydrogen concentration which corresponds to about 0.01 mol fraction olelins in the product, the catalyst is rapidly deactivated. Within the temperature range preferred for hydro-isomerization of n-pentane, from about 800 to 850 F., the feed composition which will produce the maximum permissible olelin concentration inthe product of 0.01 mol fraction can be obtained by substituting 0.01 for x in Equation l above. The minimum hydrogen to pentane mol feed ratio for avoiding rapid aging of the catalyst of Example Il can then be expressed as:

Over the preferred temperature range, a and b can be expressed as follows:

In Equations 3 and 4 T is absolute temperature in degrecs' Kelvin and P is absolute pressure in atmospheres. Substituting values of a and b for the particular temperature and pressure employed, Equation 2 can` be Very useful for determining' the feed composition to be used in hydroisomerization of n-pentane. For example, when the temperature is to be raised to maintain catalyst activity during the gradual decline of catalyst activity over 'a long-cycle run, as we discuss more fully below, Equation 2 can advantageously be used to determine the minimum feed hydrogen concentration for the higher temperature. The hydrogen concentration is thus adjusted in accordance with Equation 2 to keep the product olefin concentration substantially constant and rapid aging of the catalyst is avoided.

Examples l `and Il and their data as plotted in Figures 2 and 3 show the important practical discovery we have made with regard to the relationship between catalyst deactivation and olefin content of the isomerization product. With this discovery it is possible in accordance with our invention to control the hydroisomerization of n-pentane in such a manner as to obtain high yields of isopentane without running the danger of rapid catalyst deactivation. With our discovery it is possible to use olefin concentration of the product as an indication of the proper level of operating conditions. The hydrogen concentration of the feed is one of the variables of the process most subject to iluctuation in refinery operations. In the hydroisomerization of npentane at low hydrogen concentrations and high space velocity to obtain a high space-time-yield, any fluctuation in the direction of reduction of the hydrogen concentration can have serious consequences. As curves A and E of Figures 2 land 3 show, a slight decrease in hydrogen concentration near the peak of the space-time-yield curve can cause rapid catalyst deactivation. Furthermore, this is a permanent deactivation in that the -acivity is not restored merely by raising the hydrogen concentration. The catalyst must be regenerated by burning o'l carbonaceous deposits in order to restore its original activity after it has once been lost by operating with an excessively low hydrogen concentration. The olefin concentration of the product can be readily measured by known methods and equipment. ln this respect it contrasts sharply with the more direct indication of catalyst deactivation, namely the build-up of carbon on the catalyst, which cannot readily be measured during the process. Accordingly, we have made a valuable improvement in the control of the isomerization process through the use of the olefin concentration of the product as an indicator for setting the process variables, particularly the variable of feed concentration. At the high space velocities employed in the process of the invention the time lag between the change of feed composition and its first effect on the product emerging from the reactor is relatively short. It is sufficiently short that the steps can be taken when analysis shows that the product olefin concentration is excessive to correct the feed composition before catalyst deactivation has gone too far.

ln accordance with the principles discussed above, an important advantage of our process is to provide a means for correcting fluctuations in the hydrogen concentration of the feed which might cause catalyst deactivation. Thus, if for any reason the hydrogen flow rate to the hydroisomerization reactor should drop either suddenly or gradually, the result will be an increase in olefin concentration in the product. When product control measurements indicate this, the feed composition can be corrected by increasing hydrogen iiow rate -or decreasing the hydrocarbon flow rate.

In using a long-life platinumralumina catalyst for hydroisomerization, the activity of the catalyst gradually declines even though the process is carried out with a hydrogen concentration sul'lciently` high to prevent rapid catalyst deactivation. Accordingly the space-time-yield of isopentane gradually decreases. The proper procedure under such circumstances is to increase the reaction ternperature sufficiently to increase the yield of isopentane to the desired level. However, if the temperature is raised without changingl other process variables, such as feed composition, the olefin concentration of the product will increase and in a short time the catalyst activity will decline rapidly. Thus, when the temperature is raised to maintain yields `it is also necessary to raise the hydrogen concentration in order to avoid catalyst deactivation. In accordance with the invention, when the temperature is periodically raised, the hydrogen concentration of the feed is also raised in response to measurement of the olefin concentration of the product so as to maintain the olefin concentration substantially at the same level `as was obtained before the temperature was increased. By maintaining a substantially constant olefin concentration in the product the yield of isopentane can be maintained at a high level Without excessively aging the catalyst. Therefore, one modification of our process comprises the procedure of raising reaction temperature when product yield declines below a desired level, measuring the olefin content of the isomerization product and raising the hydrogen concentration of the feed in response to changes in the olefin concentration of the product whereby to maintain the olefin concentration substantially constant.

We have discussed the measuring of product olefin concentration in our process. This can be done in a number of different ways. Thus, in the runs of Examples I and II the product olefin concentrations were determined from bromine numbers of the isomerization products obtained in accordance with ASTM Test Dl159. Various other conventional batch or continuous analysis techniques can also be used. In refinery scale operations of our process it is particularly advantageous to use a continuous analyzing technique such as analysis by continuous spectrometry. The particular means to be used for this purpose is not a part of our invention. In our process a number of commercial analyzing control instruments can be adapted for continuously measuring the olefin content of the product and controlling hydrogen feed rate in response to changes in the olefin content. Instruments that can be adapted for this use include an automatic differential refractometer which continuously measures deviations of refractive index of a liquid stream. A mass spectrometer adapted for continuous analysis and provided with intermediate scanning means can also be used. A combination of several analyzing-control instruments employing continuous infrared absorption spectrometry and having means for totaling the measurements for each of the several olefins in the product can also be adapted for use in the process.

We will now describe an apparatus in which the process of the invention can be carried out. Suitable apparatus is shown diagrammatically in Figure 1 of the drawing. The apparatus comprises a hydroisomerization reactor 30 containing a fixed bed of supported platinum catalyst particles. The n-pentane feed is charged to the system via line 31. It passes through the feed preheater 32 where it is heated to reaction temperature and then passes via line 33 into the reactor 30. Fresh hydrogen in a regulated amount is charged to the system via line 34. The hydrogen also is heated in the preheater 32 and the heated hydrogen stream is mixed with the vaporized pentane stream in line 33 before entering reactor 30. The isomerization product is withdrawn from reactor 30 via line 35 which passes through an analyzing-control means 36 lwhich is adapted to measure small olen concentrations in the product stream and transmit control impulses to a valve control means. In the embodiment of the drawing, this means takes the form of a controller 37 pneumatically connected with a diaphragm control for valve 38 in the hydrogen line 34. After passing through the analyzer the isomerization product stream passes through a condenser 39 wherein normally liquid hydrocarbons are condensed. The cooled product then passes to the separator 40 where uncondensed gases are separated from the isomerzed pentane product.' An analyzing-controller such as a continuous refractometer which analyzes liquid jmixtures would be placed in the product line after the condenser 39 or in the liquid withdrawal line from separator 40 instead of before the condenser, as shown in the drawing. The uncondensed gases recovered from the separator 40 via line 41 are preferably passed through a light oil scrubber, not shown in the drawing, to reduce the hydrocarbon content of the gas. The remaining hydrogen is then recycled via line 42 to the hydrogen inlet line 34.

We have described our invention with regard to isomerization runs carried out with specific catalysts and under specific operating conditions. However, the principles of the invention involving the control of process variables in response to changes in the olefin content of the isomerization product apply in general to the types of catalysts and ranges of process conditions for isomerizing npentane which are disclosed in the Stai-nes and Zabor patent application Serial No. 508,980, led May 17, 1955. Thus, the principles of the invention apply in general to the control of n-pentane hydroisomerization using a high space velocity and a low feed hydrogen concentration wherein minor variations in hydrogen concentration can seriously affect space-time-yield of isomer product or catalyst life.

Ranges of conditions described in th mentioned patent application include feed ratios of hydrogen to hydrocarbon of yfrom about 50 to 1000 standard cubic feet of hydrogen per barrel of hydrocarbon in the charge. Stated in other units, this range is from about 0.96 to 0.53 mol fraction of hydrocarbon in the charge. It should be understood, however, that within this range the minimum permissible hydrogen concentration to avoid rapid catalyst deactivation may, for a particular catalyst and particular operating conditions, be somewhat greater than the lower limit of the range. Reaction pressures for the type of process to which the invention applies range from about 50 to 1000 pounds per square inch gauge. The temperature can range from 700 to l000 F. The space Velocity, which is higher than is used in naphtha reforming, is above 5 liquid volumes of charge per volume of catalyst per hour. Normally, the space velocity will be above 8 volumes per volume per hour and in some operations it may be as high as 20 or more volumes per volume per hour.

The catalysts used in the type of process to which the invention applies are supported platinum catalysts. Such catalysts comprise a minor amount of platinum and a major amount of a supporting material. The catalyst can be in the form of irregular granules or of particles of uniform size and shape prepared by pilling, extrusion or other suitable methods. Generally, the platinum content is 0.1 to 5.0 percent by weight and preferably is about 0.2 to 1.0 percent by weight. The preferred support or carrler is alumina. Other suitable supports or carriers include silica-stabilized alumina; fresh, aged or deactivated silica-alumina composites; silica-magnesia; bauxite and the like. The catalyst preferably contains minor amounts, for example from 0.1 to 10 percent by Weight, of combined halogen and/or other activating components. Halogen compounds or other activating components can also be included in the feed to the process.

Obviously many modifications and variations of the in- Ventlon as hereinbefore set forth may be made without departing from the spirit and scope thereof and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. A hydroisomerization process which comprises contacting in admixture with hydrogen a predominantly n-pentane hydrocarbon fraction with a supported platinum catalyst at a temperature from 700 to l000 F. 'and a liquid-hourly space velocity of at least 5 volumes of hydrocarbon per volume of catalyst per hour, the concentration of hydrocarbon in admixture with said hydrogen being less than the concentration which corresponds to rapid catalyst aging at the isomerization conditions employed, recovering an isomerization product, `measuring the olefin concentration of said product and adjusting the hydrogen concentration in admixture with said hydrocarbon fraction in response to changes in said olefin concentration whereby to maintain said olefin concentration at a level corresponding to high space-time-yield of isopentane but below the olefin concentration corresponding to rapid catalyst deactivation.

2. A hydroisomerization process which comprises contacting in admixture with hydrogen a predominantly n-pentane hydrocarbon fraction with a supported platinum catalyst at a temperature from 700 to 1000 F. and a liquid-hourly space velocity of at least volumes of hydrocarbon per volume of catalyst per hour, the concentration of hydrocarbon in admixture with said hydrogen being greater than a mol fraction of 0.5 and less than the concentration which corresponds to rapid catalyst aging at the isomerization conditions employed, recovering an isomerization product, measuring the olefin concentration of said product and increasing the hydrogen concentration in admixture with said hydrocarbon fraction in response to changes in said olen concentration whereby to maintain said olefin concentration substanitally constant at a level below the level corresponding to rapid catalyst deactivation.

3. A hydroisomerization process which comprises contacting in admixture with hydrogen a predominantly n-pentane hydrocarbon fraction with a supported platinum catalyst at a temperature from 700 to 1000 F. and a liquid-hourly space velocity of at least 5 volumes of hydrocarbon per Volume of catalyst per hour, the concentration of hydrocarbon in admixture with said hydrogen being greater than a mol fraction of 0.5 and less than the concentration which corresponds to rapid catalyst aging at the isomerization conditions employed, recovering an isomerization product, measuring the olen content of said product, raising the reaction temperature when the rate of conversion of the hydrocarbon charge decreases below a desired level and `simultaneously raising the hydrogen concentration of the charge mixture in an amount sufficient to maintain substantially constant the olen content of the isomerization product.

4. A hydroisomerization process which comprises contacting a feed comprising hydrogen and n-pentane with a supported platinum catalyst at a temperature from 700 to l000 F. and a liquid-hourly space velocity of at least 5 volumes of hydrocarbon per volume of catalyst per hour, the concentration of hydrocarbon in said feed being greater than a mol fraction of 0.5 and less than the concentration which corresponds to rapid catalyst aging at the isomerization conditions employed, recovering an isomerization product, measuring the olefin content of said product, periodically raising the reaction temperature when the rate of conversion of the hydrocarbon charge decreases below a desired level whereby to restore said rate of conversion substantially to its original level, and maintaining substantially constant the olefin content of the isomerization product when said temperature is raised by simultaneously raising the hydrogen concentration of the feed to a value as expressed by the following equation:

l0 wherein r is the mol ratio of hydrogen to pentane in the feed; x is the mol fraction of penteues in the isomerization product;

and

Tbeing the absolute temperature in degrees Kelvin and P being the absolute pressure in atmospheres.

5. A hydroisornerization process which comprises contacting a feed comprising hydrogen and n-pentane with a catalyst consisting essentially of 0.2 to 1.0 weight percent platinum, 0.1 to 10 percent halogen, and the balance alumina, at a temperature from 800 to 850 F., a pressure from 50 to 1000 pounds per square inch gauge, and a liquid hourly space Velocity of at least 8 volumes of hydrocarbon per volume of catalyst per hour, the concentration of hydrocarbon in said feed being greater than a mol fraction of 0.5 and less than the concentration which corresponds to rapid catalyst aging at the isomerization conditions employed, recovering an isomerization product, measuring the olefin concentration of said product and maintaining the olefin concentration of said product substantially constant at about 1.0 mol percent of olefin based on the total hydrocarbon product by using reaction conditions of temperature, pressure and mol ratio of hydrogen to hydrocarbon in the feed in accordance with the following equation:

wherein r is the mol ratio of hydrogen to pentane in the feed;

and

5900 b =w 10 7 T T being the absolute temperature in degrees Kelvin and P the absolute pressure in atmospheres.

References Cited in the file of this patent UNITED STATES PATENTS 2,462,946 Coggeshall et al Mar. 1, 1949 2,479,110 Haensel Aug. 16, 1949 2,550,531 Ciapetta Apr. 24, 1951 2,642,384 `Cox June 16, 1953 2,645,605 Lang et al July 14, 1953 2,736,684 Tarnpoll Feb. 28, 1956 2,831,908 Starnes et al Apr. 22, 1958 OTHER REFERENCES Randall et al.: Infrared Determination of Organic Structures, page 52, Van Nostrand Co. Inc., 1949. 

1. A HYDROISOMERIZATION PROCESS WHICH COMPRISES CONTACTING IN ADMIXTURE WITH HYDROGERN A PREDOMINANTLY N-HEPTANE HYDROCARBON FRACTION WITH A SUUPPORTED PLATINUM CATALYST AT A TEMPERATURE FROM 700* TO 1000*F. AND A LIQUID-HOURLY SPACE VELOCITY OF AT LEASR 5 VOLUMES OF HYDROCARBON PER VOLUME OF CATLYST PER HOUR, THE CONCENTRATION OF HYDROCARBON IN ADMIXTURE WITH SAID HYDROGEN BEING LESS THAN THE CONCENTRATION WHICH CORRESPONDS TO RAPID CATALYST AGING AT THE ISOMERIZATION CONDITIONS EM- 